First, the Pantera is NOT a racer. It is a Grand Touring street machine. IMHO, unless you are racing for money, swapping the ZF for anything else will be very costly. First, the frame rails which mount the engine & transaxle are quite narrow and also mount the rear suspension a-arms. So with something like a Ricardo 6-speed transaxle (Ford GT), the case is so wide you'll need to spread the rear frame rails apart for clearance. Now the gearbox fits but you've changed all of Dallara's carefully designed rear suspension geometry.
After that's somehow straightened out so the car handles at least as good as it did as-stock, getting the shifter stick in the cabin to work the gearbox reliably may be a challenge. The ZF 5-speed in other than a pro-racing venue, is known to reliably handle at least 750 bhp and in one example- 900 hp for 90 mile SS races. As long as one does not shift like an animal.... Beyond about 550 bhp, you will likely need a whole different car. And a deep-pocket sponsor.